In the electronics industry, electrical components such as resisters, capacitors, inductors, transistors, integrated circuits, chip carriers and the like, are typically mounted on circuit boards in one of two ways. In the first way, the components are mounted on one side of the board and leads from the components extend through holes in the board and are soldered on the opposite side of the board. In the second way, the components are soldered to the same side of the board upon which they are mounted. These latter devices are said to be "surface-mounted."
Surface mounting of electronic components is a desirable technique in that it may be used to fabricate very small circuit structures and lends itself well to process automation. One family of surface-mounted devices, referred to as "flip chips", comprises integrated circuit devices having numerous connecting leads attached to pads mounted on the underside of the device. In connection with the use of flip chips, either the circuit board or the chip is provided with small balls of solder positioned in locations which correspond to the pads on the underside of each chip and on the surface of the circuit board. The chip is mounted by (a) placing it in contact with the board such that the solder balls become sandwiched between the pads on the board and the corresponding pads on the chip; (b) heating the assembly to a point at which the solder is caused to reflow (i.e., melt); and (c) cooling the assembly. Upon cooling, the solder hardens, thereby mounting the flip chip to the board's surface. Tolerances in devices using flip chip technology are critical, as the spacing between individual devices as well as the spacing between the chip and the board is typically very small. For example, spacing of such chips from the surface of the board is typically in the range of 0.5-3.0 mil and is expected to approach micron spacing in the near future.
One problem associated with flip chip technology is that the chips, the solder and the material forming the circuit board often have significantly different coefficients of thermal expansion. As a result, differing expansions as the assembly heats during use can cause severe stresses, i.e., thermomechanical fatigue, at the chip connections and can lead to failures which degrade device performance or in capacitate the device entirely.
In order to minimize thermomechanical fatigue resulting fro m different thermal expansions, thermoset epoxies have been used. Specifically, these epoxies are used as an underflow material which surrounds the periphery of the flip chip and occupies the e space beneath the chip between the underside of the chip and the board which is not occupied by solder. Such epoxy systems provide a level of protection by forming a physical barrier which resists or reduces different expansions among the components of the device .
Improved underflow materials have been developed in which the epoxy thermoset material is provided with a silica powder filler. By varying the amount of filler material, it is possible to cause the coefficient of thermal expansion of the filled epoxy thermoset to match that of the solder. In so doing, relative movement between the underside of the flip chip and the solder connections, resulting from their differing coefficients of thermal expansion, is minimized. Such filled epoxy thermosets therefore reduce the likelihood of device failure resulting from thermomechanical fatigue during operation of the device.
At high levels of filler, (i.e., about 70% filler), it becomes very difficult to fully wet out the silica. As a result, highly filled epoxy systems often include a wetting agent, such as an epoxy silane, to enhance the ability of the resin to wet the filler particles. Unfortunately, such wetting agents can be adversely affected by accelerants which are provided to speed the curing of the epoxy system. As a result, it becomes very difficult to design an epoxy system which adequately wets a filler material, and cures within a commercially acceptable time period. Additionally, to date, fast curing epoxy systems have not been practical because the useful pot life of the material becomes significantly reduced as cure time decreases.